1. Field of the Invention
The present invention relates to methods for manufacturing an acoustic wave device and, more specifically, to methods for manufacturing an acoustic wave device that includes a piezoelectric substrate, an IDT electrode located on the piezoelectric substrate, and a dielectric film arranged to cover the IDT electrode.
2. Description of the Related Art
Cellular phones and other communication devices usually have a built-in acoustic wave device in their RF (radio frequency) circuit and this device works as a duplexer, an interstage filter, or the like. There are several types of acoustic wave devices available, including those based on surface acoustic waves (surface acoustic wave devices) and those based on boundary acoustic waves (boundary acoustic wave devices).
The acoustic wave device has an IDT electrode on its piezoelectric substrate, and this IDT electrode is driven to generate acoustic waves. The piezoelectric substrate is made of LiTaO3, LiNbO3, or any other similar compound. Acoustic wave devices with a piezoelectric substrate made of these compounds have a negative temperature coefficient of frequency (TCF). Specifically, LiNbO3 substrates have TCFs on the order of −90 ppm/° C. to −70 ppm/° C., and LiTaO3 substrates have TCFs on the order of −40 ppm/° C. to −30 ppm/° C. As a result, acoustic wave devices based on these substrates often fall short of the required frequency-temperature profile that duplexers and interstage filters should have.
A solution to this kind of problem can be found in Japanese Unexamined Patent Application Publication No. 2006-254507, which proposes an acoustic wave device that has a dielectric film in addition to a piezoelectric substrate made of LiTaO3, LiNbO3, or any other similar compound and an IDT electrode formed on the piezoelectric substrate. This dielectric film has a positive TCF and is formed to cover the IDT electrode. The chemical structure of the dielectric film is mainly composed of Si and O and has some bonds substituted with a hydrogen atom, a fluorine atom, and/or a hydroxy group (OH). According to Japanese Unexamined Patent Application Publication No. 2006-254507, this constitution leads to an excellent frequency-temperature profile.
However, the acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2006-254507 is disadvantageous in that its electrical properties may be affected depending on the method chosen for the formation of the dielectric film.